<!DOCTYPE html>
<html lang="en">

<head>
    <meta charset="UTF-8">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>Document</title>
    <style>
        input {
            color: #999;
        }
    </style>
</head>

<body>
    <input type="text" value="手机" name="" id="">
    <script>
        var text = document.querySelector('input');
        //获得焦点事件 onfocus
        text.onfocus = function () {
            if (this.value === '手机') {
                this.value = '';
            }
            this.style.color = '#333';
        }
        //失去焦点 onblur
        text.onblur = function () {
            if (this.value === '') {
                this.value = '手机';
            }
            this.style.color = '#999'
        }
    </script>
</body>

</html>